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    // Postorder Traversal
    public List<Integer> postorder(Node root) {
        List<Integer> result = new ArrayList<>();
        postorderHelper(root, result);
        return result;
    }

    private void postorderHelper(Node node, List<Integer> result) {
        if (node == null) return;
        for (Node child : node.children) { // Process children
            postorderHelper(child, result);
        }
        result.add(node.val); // Process root
    }